An Introduction to Muscle Tissue Muscle Tissue One
An Introduction to Muscle Tissue Muscle Tissue One of the four primary tissue types, divided into: Skeletal muscle tissue Cardiac muscle tissue Smooth muscle tissue 2012 Pearson Education, Inc. 10-1 Properties of Muscles Excitability: capacity of a muscle to respond to a stimulus Conductivity: ability to propagate electrical signals over membrane Contractility: ability of a muscle to shorten or exhibit force Extensibility: muscle can be stretched beyond its normal resting length Elasticity: ability of muscle to recoil after being stretched beyond
its normal length 2012 Pearson Education, Inc. 10-1 Functions of Skeletal Muscle Tissue Six Functions of Skeletal Muscle Tissue 1. Produce skeletal movement 2. Maintain posture and body position 3. Support soft tissues 4. Guard entrances and exits 5. Maintain body temperature 6. Store nutrient reserves 2012 Pearson Education, Inc. 10-2 Organization of Muscle Skeletal Muscles contain
Muscle tissue (muscle cells or fibers) Connective tissues Nerves Blood vessels 2012 Pearson Education, Inc. Organization of Connective Tissues Muscles have three layers of connective tissues 1. Epimysium: exterior layer, separates muscle from surrounding tissue 2. Perimysium: surrounds each fascicle (bundles of muscle fibers). Contains blood vessel and nerve 3. Endomysium: Surrounds individual muscle cells (fibers). Contains capillaries, nerve fibers & myosatellite cells (stem
cells)-involved in repair Endomysium, perimysium, and epimysium come together at ends of muscles to form a Tendon (bundle) or aponeurosis (sheet)Attaches muscle to bone 2012 Pearson Education, Inc. Figure 10-1 The Organization of Skeletal Muscles Skeletal Muscle (organ) Epimysium Perimysium Muscle fascicle Endomysium
Nerve Muscle Blood fibers vessels Epimysium Blood vessels and nerves Tendon Endomysium Perimysium 2012 Pearson Education, Inc. Blood Vessels and Nerves
Blood: each skeletal muscle receives blood from one artery and is drained by two veins Nerves: a single nerve supplies each skeletal muscle with information from the brain. Each nerve is composed of many motor neurons (cells). Motor neuron axons branch many times, each extending to a different skeletal muscle fiber 2012 Pearson Education, Inc. Skeletal Muscle Cell Development During development, groups of mesodermal cells (myoblasts) fuse to form multinucleate muscle fibers Unfused myoblasts are myosatellite cells; help repair muscle tissue
2012 Pearson Education, Inc. The Formation of a Multinucleate Skeletal Muscle Fiber Muscle fibers develop through the fusion of mesodermal cells called myoblasts. Myoblasts A muscle fiber forms by the fusion of myoblasts. LM 612
Muscle fiber Sarcolemma Nuclei Myofibrils Myosatellite cell Nuclei Immature muscle fiber Mitochondria Myosatellite cell A diagrammatic view and a
micrograph of one muscle fiber. Up to 30 cm in length Mature muscle fiber 2012 Pearson Education, Inc. 10-3 Characteristics of Skeletal Muscle Fibers Skeletal Muscle Cells: myofibers Are very long Multinucleate: Contain hundreds of nuclei Contain numerous mitochondria to provide ATP for contraction and relaxation Contain glycogen as glucose reserve for energy production Contain myoglobin as oxygen reserve
2012 Pearson Education, Inc. Muscle Fiber Sarcoplasm (cytoplasm of muscle fiber): Contains large amount of glycogen as a glucose source and myoglobin that stores oxygen Mitochondria: throughout the cell, near the contractile proteins. The fiber needs ATP to keep the cell prepared for contraction and for the contraction itself. 2012 Pearson Education, Inc. Muscle Fiber Sarcolemma and Transverse Tubules Sarcolemma is the cell (plasma) membrane of a muscle fiber Important for transmission of an action potential in the muscle fiber
Transverse tubules (T tubules) Narrow tubes that are continuous with sarcolemma These infoldings of the sarcolemma are important for transmission of action potential from the sarcolemma through the interior of the cell Allow entire muscle fiber to contract simultaneously 2012 Pearson Education, Inc. Figure 10-3 The Structure of a Skeletal Muscle Fiber Myofibril Sarcolemma Nuclei Sarcoplasm
2012 Pearson Education, Inc. Muscle Fiber Sarcoplasmic Reticulum (SR) Surrounds each myofibril Similar in structure to smooth endoplasmic reticulum Stores Calcium ions- Ca2+ when muscle is relaxed Encircled by T tubules Terminal cisternae Expanded chambers of the SR on either side of the T tubule Concentrate Ca2+ (via ion pumps) A muscle contraction begins when Ca2+ is released from the terminal cisternae Two terminal cisterns and a T-tubule are a triad. 2012 Pearson Education, Inc.
The combination of one T tubule and two terminal cisternae is called a Triad 2012 Pearson Education, Inc. Muscle Fiber Myofibrils and Myofilaments Lengthwise subdivisions within muscle fiber As long as muscle cell but much smaller in diameter; 100s1000s per cell Made up of bundles of protein filaments called myofilaments responsible for muscle contraction have repeating functional units called sarcomeres
Consist of thin & thick filaments 2012 Pearson Education, Inc. Figure 10-3 The Structure of a Skeletal Muscle Fiber Myofibril Sarcolemma Nuclei Sarcoplasm MUSCLE FIBER Mitochondria
Sarcolemma Sarcolemma Sarcoplasm Myofibril Myofibrils Thin filament Thick filament Sarcoplasmic T tubules reticulum 2012 Pearson Education, Inc. Muscle Fiber Sarcomeres
Filaments are arranged in repeating units called sarcomeres, separated by Z discs (lines). The sarcomere is the structural and functional unit of skeletal muscle. Thick and thin filaments overlap each other in a pattern that creates striations Consist of dark A bands and light I bands 2012 Pearson Education, Inc. Sarcomere: The I Band Contains thin filaments but no thick filaments Has Z lines: The centers of the I bands; Also mark the boundary between adjacent sarcomeres-Composed of proteins to anchor thin filaments I band
A band H band Z line Titin A longitudinal section of a sarcomere, showing bands Zone of overlap 2012 Pearson Education, Inc. M line Thin Thick
filament filament Sarcomere: The A Band Dense region of the sarcomere that contains thick or thick & thin filaments. Consists of: M line, H band : Thick filaments only Zone of overlap: Thick and Thin filaments I band A band H band
Z line Titin A longitudinal section of a sarcomere, showing bands Zone of overlap 2012 Pearson Education, Inc. M line Thin Thick filament filament Thin Filaments
1. Actin : Two twisted rows of filamentous actin (F-actin) containing globular actin (G-actin) subunits The active sites on actin strands bind to myosin during muscle contraction 2. Tropomyosin: A double stranded chain that wraps around the actin chain Covers the active sites of G-actin; hence prevents actinmyosin interaction 3. Troponin: A globular protein with three subunits One subunit binds tropomyosin: forms a troponin/tropomyosin complex One subunit binds to actin; holding the troponin-tropomyosin in place One subunit binds Ca2+; Resting cell Ca2+ concentration is low so binding
site is empty Contraction controlled by calcium binding to troponin 2012 Pearson Education, Inc. Figure 10-7ab Thick and Thin Filaments Sarcomere H band Actinin Z line Titin Myofibril The gross structure of a thin filament, showing the attachment at the Z line
Z line M line Troponin Active site Tropomyosin G-actin molecules F-actin strand The organization of G-actin subunits in an F-actin strand, and the position of the troponintropomyosin complex 2012 Pearson Education, Inc.
Thick Filaments: Myosin Myosin Head region: Made of two globular protein subunits Binding site for actin Binding and catalytic site for ATP: Split into ADP and Pi by ATPase enzyme activity Causes head to be energized During contraction, myosin interacts with the thin filament, forming crossbridges with actin Myosin Tail region: Binds to other myosin molecules: Points towards M line Titin The structure of thick filaments, showing the orientation of the myosin molecules 2012 Pearson Education, Inc.
M line Myosin head Myosin tail The structure of a myosin molecule Sarcomere Structure Titin are strands of elastic protein that reach from tips of thick filaments to the Z line I band
Keeps thick and thin filaments in alignment Recoil after stretching; Aids in restoring sarcomere to proper length after contraction A band H band Z line Titin A longitudinal
section of a sarcomere, showing bands Zone of overlap M line Sarcomere 2012 Pearson Education, Inc. Thin Thick filament filament Sarcomere Structure I band
A band H band A corresponding view of a sarcomere in a myofibril from a muscle fiber in the Myofibril gastrocnemius Z line muscle of the calf Z line TEM 64,000 Zone of overlap M line
Sarcomere http://wps.aw.com/bc_marieb_ehap_9/79/20308/5199051.cw/index.html 2012 Pearson Education, Inc. 10-4 Components of the Neuromuscular Junction Skeletal muscle fibers begin contraction when signals from nervous system cause them to release their internal calcium stores. Communication of nervous and muscle fibers occurs at special intercellular connections called the neuromuscular junction (NMJ) 2012 Pearson Education, Inc. 10-4 Components of the Neuromuscular Junction
Neuromuscular junction (NMJ) has three parts: 1. Synaptic terminal (synaptic end knobs) of axon of motor neuron 2. Synaptic cleft 3. Motor end plate of muscle fiber 2012 Pearson Education, Inc. https://www.youtube.com/watch?v=7wM5_aUn2qs Figure 10-11 Skeletal Muscle Innervation The cytoplasm of the synaptic terminal contains vesicles filled with molecules of acetylcholine, or ACh. Acetylcholine is a
neurotransmitter, a chemical released by a neuron to change the permeability or other properties of another cells plasma membrane. The synaptic cleft and the motor end plate contain molecules of the enzyme acetylcholinesterase (AChE), which breaks down ACh. Vesicles The synaptic cleft, a narrow space, separates the synaptic terminal of the neuron from the opposing motor end
plate. 2012 Pearson Education, Inc. Junctional AChE fold of motor end plate ACh Figure 10-11 Skeletal Muscle Innervation The stimulus for ACh release is the arrival of an electrical impulse, or action potential, at the synaptic terminal. Arriving action
potential 2012 Pearson Education, Inc. Figure 10-11 Skeletal Muscle Innervation When the action potential reaches the neurons synaptic terminal, permeability changes in the membrane trigger the exocytosis of ACh into the synaptic cleft. Exocytosis occurs as vesicles fuse with the neurons plasma membrane. Motor end plate
2012 Pearson Education, Inc. Figure 10-11 Skeletal Muscle Innervation ACh molecules diffuse across the synatpic cleft and bind to ACh receptors on the surface of the motor end plate. ACh binding triggers the generation of an action potential in the myofiber that spreads along the sarcolemma. Myofiber is excited. ACh receptor site
2012 Pearson Education, Inc. Figure 10-11 Skeletal Muscle Innervation AChE quickly breaks down the ACh on the motor end plate and in the synaptic cleft, thus inactivating the ACh receptor sites. Action potential AChE 2012 Pearson Education, Inc.
Table 10-1 Steps Involved in Skeletal Muscle Contraction and Relaxation Steps in Initiating Muscle Contraction Motor Synaptic terminal end plate T tubule Sarcolemma Action potential reaches T tubule ACh released, binding
to receptors Sarcoplasmic reticulum releases Ca2 Active site exposure, cross-bridge formation Contraction begins 2012 Pearson Education, Inc. Steps in Muscle Relaxation The
link between generation of an action potential in the sarcolemma and the start of a muscle contraction is called Excitation Contraction Coupling ACh broken down by AChE Action potential in sarcolemma travels along Sarcoplasmic reticulum recaptures the TCa tubules to the triads Triggers the releases of Active sites covered, 2+ Ca nofrom the terminal
cross-bridge interaction cisternae Contraction Ca2+ is released at the zones ends of overlap, where thick and thin filaments overlap Relaxation occurs, passive return to resting length Initiates contraction 2 Ca2
Actin Myosin Figure 10-12 The Contraction Cycle Contraction Cycle Begins Release of calcium from SR at the zone of overlap. Myosin head Troponin Tropomyosin 2012 Pearson Education, Inc. Actin
Figure 10-12 The Contraction Cycle Active-Site Exposure Calcium ions bind to troponin, weakening the bond between actin and the troponin tropomyosin complex. The troponin molecule then changes position, rolling the tropomyosin molecule away from the active sites on actin and allowing interaction with the energized myosin heads. Sarcoplasm
Active site 2012 Pearson Education, Inc. Figure 10-12 The Contraction Cycle Cross-Bridge Formation Once the active sites are exposed, the energized myosin heads bind to them, forming cross-bridges. 2012 Pearson Education, Inc. Figure 10-12 The Contraction Cycle
Myosin Head Pivoting After cross-bridge formation, the energy that was stored in the resting state is released as the myosin head pivots toward the M line. This action is called the power stroke; when it occurs, the bound ADP and phosphate group are released. 2012 Pearson Education, Inc. Figure 10-12 The Contraction Cycle Cross-Bridge Detachment When another ATP binds to
the myosin head, the link between the myosin head and the active site on the actin molecule is broken. The active site is now exposed and able to form another cross-bridge. 2012 Pearson Education, Inc. Figure 10-12 The Contraction Cycle Myosin Reactivation Myosin reactivation occurs when the free myosin head splits ATP into ADP and P. The
energy released is used to recock the myosin head. 2012 Pearson Education, Inc. Sliding Filaments and Muscle Contraction What happens when a skeletal muscle fiber contracts? Sliding filament theory: Observations The H and I bands of the sarcomere get smaller The zones of overlap get larger Z lines move closer together The width of A zone stays the same 2012 Pearson Education, Inc. Sliding Filaments and Muscle Contraction
What happens when a skeletal muscle fiber contracts? Sliding filament theory: Explanation Thin filaments of sarcomere slide toward center (M line) of each sarcomere, alongside thick filaments 2012 Pearson Education, Inc. A relaxed sarcomere I band Z line 2012 Pearson Education, Inc. A band
H band Z line Figure 10-8b Changes in the Appearance of a Sarcomere during the Contraction of a Skeletal Muscle Fiber I band A band Z line H band A contracted sarcomere A band stays the same width Z lines move closer together
Zones of overlap get bigger H & I band gets smaller 2012 Pearson Education, Inc. Z line Sliding Filaments and Muscle Contraction During a contraction, thick and thin filaments slide past each other, pulling the Z lines behind them Sliding occurs in every sarcomere along the myofibril - Myofibril gets shorter Myofibrils attached to sarcolemma at each Z line - Muscle fiber also gets shorter 2012 Pearson Education, Inc.
Contraction Contraction Duration depends on: Duration of neural stimulus Number of free calcium ions in sarcoplasm Availability of ATP 2012 Pearson Education, Inc. Relaxation Nervous stimulation stops: Ach broken down by AChE does not generate any new action potential in the myofiber Ca2+ detaches from troponin Active sites are re-covered by tropomyosin Ca2+ is pumped back into the SR or out of the cell by active transport that requires ATP In the absence of calcium, tropomyosin/troponin complex
prevent Actin/Myosin Interaction Thin myofilaments slide back to their original relaxed position 2012 Pearson Education, Inc. Muscle Return to Resting Length Elastic Forces The pull of elastic elements (tendons, ligaments) Expands the sarcomeres to resting length Opposing Muscle Contractions Reverse the direction of the original motion Are the work of opposing skeletal muscle pairs Gravity Can take the place of opposing muscle contraction to return a muscle to its resting state 2012 Pearson Education, Inc.
10-5 Tension Production and Contraction Types Tension Production by Muscles Fibers Force (tension) produced by muscle contraction depends on the frequency of stimulation and number of neurons sending stimulation The Frequency of Stimulation A single neural stimulation produces a single contraction or twitch which lasts about 7100 msec. Sustained muscular contractions require many repeated stimuli 2012 Pearson Education, Inc. Figure 10-15b Myogram: The Development of Tension in a Twitch Tension
Maximum tension development Stimulus Resting Latent Contraction phase period phase Relaxation phase Myogram for a single twitch in the gastrocnemius muscle. 2012 Pearson Education, Inc.
Figure 10-15a The Development of Tension in a Twitch Eye muscle Gastrocnemius Tension Soleus Stimulus Time (msec) A myogram showing differences in tension over time for a twitch in different skeletal muscles.
2012 Pearson Education, Inc. The Motor Unit Motor unit: One motor neuron and all the skeletal myofibers it innervates (stimulates) Motor neuron may control a few to to 2,000 fibers/motor unit. Small motor units found in muscles needed to perform fine skills; Largest motor units found in postural muscles
Causes a simultaneous contraction of all the muscle fibers that are connected to that motor neuron Total strength of contraction in a muscle depends on how many motor units are activated and the size of the motor units 2012 Pearson Education, Inc. Recruitment Recruitment: motor units in a whole muscle are stimulated asynchronously: some fibers active while others are relaxed. Delays muscle fatigue during sustained contraction Produces smooth contraction
2012 Pearson Education, Inc. Muscle tone The resting sustained tension in a skeletal muscle is called muscle tone Muscle with muscle tone: firm and solid Muscle without muscle tone: limp and flaccid Unconscious nerve impulses maintain the muscles in a partially contracted state; but not enough tension to produce movement Muscle tone helps stabilize the position of the bones and joints Increasing muscle tone increases metabolic energy used, even at rest 2012 Pearson Education, Inc.
Muscle contractions may be Isotonic or Isometric Isotonic contraction: Muscle tension greater than the load. Myofilaments are able to slide past each other during contractions. The muscle shortens and movement occurs Example: walking, lifting a book off your desk Isometric contraction: muscle produces force, but no movement occurs Muscle does not shorten Example: maintaining posture, supports object in a fixed position, or in attempting to lift an object that is too heavy 2012 Pearson Education, Inc. Muscle Metabolism
Plentiful supply of ATP needed To fuel active transport during relaxation To provide energy for contraction cycle Three sources of ATP production Creatine phosphate (unique to muscle fibers) Aerobic cellular respiration Anaerobic cellular respiration 2012 Pearson Education, Inc. Creatine Phosphate Excess ATP within resting muscle used to form creatine phosphate Three- six times more plentiful than ATP within
muscle Can be quickly broken down and its bond energy used to produce ATP Sustains maximal contraction for 15 sec 2012 Pearson Education, Inc. Aerobic Cellular Respiration ATP for any activity lasting over 30 seconds
Either glucose, fatty acids, or amino acids can be used by mitochondria to produce ATP in the presence of oxygen ~ 32 ATP molecules per molecule of glucose catabolized Provides 90% of ATP energy if an activity lasts more than 10 minutes 2012 Pearson Education, Inc. Anaerobic Cellular Respiration If no O2 available, glucose is broken down into pyruvic acid during
glycolysis Can provide ATP for 30 to 40 seconds of maximal activity Only 2 ATP per molecule of glucose Pyruvic acid is converted to lactic acid. Lactic acid can be reconverted to pyruvic acid when oxygen becomes available The Cori Cycle is the removal and recycling of lactic acid by the liver Liver converts lactate to pyruvate Pyruvate can be used to generate ATP in the aerobic respiration or
converted to glucose to recharge muscle glycogen reserves 2012 Pearson Education, Inc. Figure 10-20 Muscle Metabolism Fatty acids Fatty acids Blood vessels Glucose Glucose Glycogen Glycogen
Pyruvate Mitochondria Creatine To myofibrils to support muscle contraction Resting muscle: Fatty acids are catabolized; the ATP produced is used to build energy reserves of ATP, CP, and glycogen. Moderate activity: Glucose and fatty acids are catabolized; the ATP produced is used to power contraction. Lactate
Glucose Pyruvate Glycogen Creatine Lactate To myofibrils to support muscle contraction Peak activity: Most ATP is produced through glycolysis, with lactate as a by-product. Mitochondrial activity (not shown) now provides only about one-third of the ATP consumed.
2012 Pearson Education, Inc. Source for Glucose and Oxygen in Aerobic respiration Glucose Stored as glycogen in liver and skeletal muscles Oxygen Bound to myoglobin and hemoglobin 2012 Pearson Education, Inc. Muscle Fatigue and may need an extended Recovery Period: When muscles can no longer perform at the required level of activity Factors involved in Muscle Fatigue Depletion of metabolic reserves within fibers
Low pH (lactic acid): inhibits enzyme activity Damage to sarcolemma and sarcoplasmic reticulum Muscle exhaustion and pain Fatigue of the nervous system: less signal output 2012 Pearson Education, Inc. The Recovery Period is the time required after exertion for muscles to return to normal Oxygen becomes available Mitochondrial activity resumes 2012 Pearson Education, Inc. The Oxygen Debt After exercise or other exertion: The body needs more oxygen than usual to normalize
metabolic activities: to restore ATP, CP and glycogen reserves to former levels Resulting in heavy breathing Also called excess postexercise oxygen consumption (EPOC) 2012 Pearson Education, Inc. Heat Production by Active Muscles During catabolic reactions such as anaerobic and aerobic respiration, only some of the released energy is captured by the cell the rest is lost as heat energy Active muscles produce heat Up to 70% of muscle energy can be lost as heat, raising body temperature
2012 Pearson Education, Inc. Hormones and Muscle Metabolism Growth hormone, testosterone: stimulate contractile protein synthesis and muscle enlargement Anabolic steroids similar to testosterone but numerous side effects Thyroid hormone elevate the rate of energy consumption in resting and active muscles Epinephrine (adrenaline): During crisis, increase duration of stimulation and force of contraction 2012 Pearson Education, Inc. 10-7 Types of Muscles Fibers and Endurance Force is the maximum amount of tension produced by a muscle or muscle group
Endurance is the amount of time an activity can be sustained by an individual Force and endurance depend on: The types of muscle fibers (fast, slow, intermediate) Physical conditioning 2012 Pearson Education, Inc. Marathoners versus Sprinters Runners do not usually compete at short and long distances. Natural differences in the muscles of these athletes favor sprinting or long-distance running. Leg muscles contain two main types of muscle fibers: slow fibers and fast fibers 2012 Pearson Education, Inc.
Figure 6.0 2012 Pearson Education, Inc. Marathoners versus Sprinters Slow fibers Thin fibers, generate less power Last longer More blood supply, more myoglobin, more mitochondria Aerobic: Generate ATP using oxygen Fast fibers Thick fibers, generate more power Fatigue much more quickly Less blood supply, less myoglobin, less mitochondria Can generate ATP anaerobically Intermediate fibers More closely resemble fast fibers but more blood supply more resistant
to fatigue 2012 Pearson Education, Inc. Muscle Performance and the Distribution of Muscle Fibers Red muscles Mostly slow fibers Dark (e.g., chicken legs) White muscles Mostly fast fibers Pale (e.g., chicken breast) Most human muscles contain a mixture of fiber types Slow, Fast and Intermediate Pink
2012 Pearson Education, Inc. Muscle Atrophy and Hypertrophy Hypertrophy Muscle growth from heavy training Increases diameter of muscle fibers but not number of muscle fibers Increases number of myofibrils, mitochondria, glycogen reserves Atrophy Muscle shrinkage due to lack of muscle activity Reduces muscle size, tone, and power Caused by disuse, muscle or nerve injury or nervous system disorders Initially irreversible-physical therapy- but will be irreversible with continued loss of stimulation 2012 Pearson Education, Inc.
Importance of Exercise What you dont use, you lose! Muscles become flaccid when inactive for days or weeks Muscle fibers break down proteins, become smaller and weaker With prolonged inactivity, fibrous tissue may replace muscle fibers 2012 Pearson Education, Inc. Physical Conditioning improves both power and endurance Anaerobic Training Use fast fibers Fatigue quickly with strenuous activity Improved by: frequent, brief, intensive workouts that causes muscle hypertrophy
Aerobic Training Supported by mitochondria Require oxygen and nutrients Improved by: endurance by training fast fibers to be more like intermediate fibers Cardiovascular performance Interval Training Incorporates aerobic and anaerobic activities 2012 Pearson Education, Inc. Cardiac Muscle Tissue Found in the heart wall Striated, short branching fibers Single, centrally-located nucleus
Cells connected by intercalated discs with gap junctions and desmosomes Same arrangement of thick and thin filaments as skeletal muscle Fibers contract when stimulated by their own autorhythmic fibers 2012 Pearson Education, Inc. Cardiac vs. Skeletal Muscle Cardiac muscle Relatively more sarcoplasm and mitochondria Less well-developed SR Limited Ca+2 reserves.
Calcium enters the cell from extracellular fluid during contraction Prolonged delivery of Ca+2 to sarcoplasm produces a contraction that lasts 10-15 times longer than in skeletal muscle 2012 Pearson Education, Inc. Smooth Muscle Non-striated, single centrally located nucleus,involuntary Lack T-tubules; have very little SR for Ca+2 storage-- Ca+2 must flow in from outside Protein that binds calcium ions in the sarcoplasm is calmodulin (as opposed to troponin in skeletal muscle)
This facilitates myosin-actin binding and allows contraction to occur at a relatively slow rate 2012 Pearson Education, Inc. Table 10-4 A Comparison of Skeletal, Cardiac, and Smooth Muscle Tissues 2012 Pearson Education, Inc. Regeneration of muscle tissue Skeletal muscle fibers: cannot divide after first year. Repair occurs when: satellite cells and MSCs from bone marrow can produce some new cells if not enough cells produced, fibrosis occurs Cardiac muscle fibers: cannot divide or regenerate all healing is done by fibrosis (scar formation) Smooth muscle fibers: regeneration is possible
cells can grow in size (hypertrophy) some cells (uterus) can divide by mitosis (hyperplasia) new fibers can form from stem cells in blood vessel walls 2012 Pearson Education, Inc. Aging of Muscle Tissue Beginning at age 30, skeletal muscle starts to be replaced by fat Reflexes slow Maximum strength decreases Some fibers change to slow oxidative 2012 Pearson Education, Inc. Rigor Mortis A fixed muscular contraction- body stiffening that occurs
2-7h after death and ends after 1-6 days Caused when: After death, Ca+2 ions leak out of the SR and allow myosin heads to bind to actin Since ATP synthesis has ceased, crossbridges cannot detach from actin until proteolytic enzymes begin to digest the decomposing cells. 2012 Pearson Education, Inc. Abnormal Contractions Spasm: involuntary contraction of single muscle Cramp: involuntary and painful contraction of skeletal muscles due to electrolyte imbalance, insufficient hydration, fatigue 2012 Pearson Education, Inc.
Muscle disorders and diseases Botulism: paralysis of skeletal muscles due to consumption of bacterial toxin that blocks release of Ach Tetanus: Caused by infection with Clostridium tetani that thrives in low oxygen environments Deep puncture wound-Bacteria produces toxin that affects CNS and motor neurons Toxin suppresses the inhibition of motor neuron activity leading to sustained powerful contractions Symptoms include headache, muscle stiffness, lockjaw, high mortality rate. Preventative Immunization: Tetanus shot 2012 Pearson Education, Inc. Muscle Diseases Myaesthenia gravis: progressive autoimmune disorder that causes progressive damage at the NMJ Antibodies bind and block some Ach receptors at the NMJ Results in weaker muscles that fatigue easily
Muscles of face and neck most commonly affected; Begins with double vision and difficulty swallowing. May progresses to paralysis of respiratory muscles and death Treatment: Inhibitors of acetylcholinesterase, steroids that reduce antibodies that are binding the ACh receptors More common in women 20 to 20 years of age 2012 Pearson Education, Inc. Muscle Diseases Muscular dystrophies: Inherited autoimmune disorder where the immune system blocks the receptors to ACh resulting in muscle weakness and atrophy Mutated gene on X chromosome; greater frequency in males than in females Appears by age 5 and by age 12, may be unable to walk
Most common form is Duchenne muscular dystrophy: Mutation in dystrophin gene so little of this reinforcing protein in sarcolemma. Gene therapy is hoped-for treatment 2012 Pearson Education, Inc.
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